1. Field of the Invention
This invention relates to a vehicle suspension system, and more particularly to a vehicle suspension system in which the suspension characteristics of the front and rear suspension systems are made variable to enable control the steering characteristic of the vehicle.
2. Description of the Prior Art
There has been known a vehicle suspension system which has variable damping rate shock absorbers and the suspension characteristic of which can be varied by changing the damping rates of the shock absorbers. See U.S. Pat. No. 3,827,538, for instance.
In our U.S. Pat. No. 4,555,126 (granted Nov. 26, 1985), we have proposed a vehicle suspension system in which the damping rates of the shock absorbers are separately controlled to change the suspension characteristics of the front and rear suspension systems to change the steering characteristic of the vehicle according to the running condition of the vehicle. For example, when the vehicle is traveling straight forward at a speed higher than a predetermined value, e.g., 80 km/h, the damping rate of the shock absorbers of the front suspension system is increased to set the suspension characteristic of the front suspension system to a "hard" mode and the damping rate of the shock absorbers of the rear suspension system is reduced to set the suspension characteristic of the rear suspension system to a "soft" mode, thereby holding the understeer characteristic of the vehicle. When the vehicle then begins to roll, upon taking a curve, for instance, the damping rate of the shock absorbers of the front and rear suspension systems are controlled to set the suspension characteristics of both the front and rear suspension systems to the "hard" mode, thereby enhancing the gripping force of the front and rear tires to keep the steering characteristic of the vehicle neutral. As is well known, an understeer tendency is preferred when the vehicle is running straight at a high speed in order to improve running stability, while a neutral-steer tendency is preferred when the vehicle is rolling in order to stabilize the running position during cornering.
However, if the suspension characteristics of both the front and rear suspension systems are set to the "hard" mode when the vehicle rolls at a very high speed, e.g., higher than 120 km/h, the neutral-steer tendency comes to act as the oversteer tendency as the vehicle speed increases so that even a slight external disturbance can have a great influence on the steering system, thus lowering the driving stability. From this point of view, the vehicle suspension system we proposed is disadvantageous.